For example, in an international standard video encoding system, such as MPEG (Moving Picture Experts Group) or “ITU-T H.26x”, a method of defining block data (referred to as “macroblock” from here on) as a unit, the block data being a combination of 16×16 pixels of brightness signal and 8×8 pixels of color difference signal corresponding to the 16×16 pixels of brightness signal, and compressing the block data in accordance with a motion compensation technique, and an orthogonal transformation/transform coefficient quantization technique is used.
The motion compensation technique is used to reduce the redundancy of a signal in a time direction for each macroblock by using a high correlation existing between video frames. More specifically, in accordance with the motion compensation technique, an already-encoded frame which has been encoded is stored in a memory as a reference image, and a block area which provides the smallest difference in electric power between the block area itself and the current macroblock which is a target block for the motion-compensated prediction is searched for through a search range predetermined in the reference image which is stored in the memory. In accordance with the motion compensation technique, a spatial displacement between the spatial position of the block area which provides the smallest difference electric power and the spatial position of the current macroblock is then encoded as a motion vector.
Because the above-mentioned motion vector shows not only the efficiency of prediction but also a local movement in each block between video frames in many cases, a technique of generating an interpolated image by using information about the motion vector in order to increase the number of frames per unit time (frame rate) of the video image has been developed in recent years. As the simplest model, for example, there is a method of, when generating an interpolated image located just midway between video frames with respect to time, using a value which is half of the value of a motion vector as the motion vector of the interpolated image, and compensating for movements on the basis of previous and next frames by using the motion vector. This model is based on the assumption that there is a linear movement between video frames. Because the shorter the time interval between video frames, the more easily the above-mentioned assumption is realized, and the smaller the motion vector, the more easily the above-mentioned assumption is realized, an interpolated image having up to a certain degree of quality can be generated.
However, although a motion vector shows a local movement in each block between video frames in many cases, as mentioned above, a motion vector is actually determined through a search for a block area which provides the smallest difference electric power, and therefore does not show a local movement in some cases. In such a case, a disorder occurs in an interpolated image, and the disorder is very noticeable in many cases. Therefore, the issue of how to determine a “motion vector which cannot express a movement correctly” and to what extent an original movement can be correctly estimated while excluding such a motion vector is important.
For example, the following nonpatent reference 1 and patent reference 1 disclose frame interpolation techniques of extracting motion parameters from a bitstream which is acquired by using a general-purpose video image encoding method for the purpose of pseudoly increasing the frame rate of a moving image to be reproduced by a receive side dependently upon the motion parameters.